Crude oil is a fossil-based resource used for the production of transportation fuels, heat and power, asphalt, chemicals, adhesives, pharmaceuticals, polymers, fibers and other products. The United States is a major importer of crude oil and thus is heavily reliant on foreign countries to meet demand.
The nation's dependence on imported oil has in part fueled demand for sound, renewable energy technologies. Wind and solar technologies provide renewable electricity and heat as do geothermal and tidal technologies. Lignocellulosic biomass, which is an organic material comprised of lignin, cellulose and hemicelluloses and derived from a variety of wood and biowaste feedstocks, represents the world's only renewable carbon resource for energy, fuels, chemicals and other biobased products.
Non-food biomass is an abundant and geographically-diverse, domestic resource, which the United States Department of Agriculture (USDA) estimates can be produced in quantities exceeding 1 billion dry tons annually (see Perlack, R. D.; Stokes, B. J.; et al. Biomass as feedstock for a bioenergy and bioproducts industry: The technical feasibility of a billion-ton annual supply, USDA-DOE, 2005). As noted above, biomass resources include agricultural residues such as corn stover, forestry and mill residues, wood, energy crops such as switchgrass, miscanthus, energy cane, algae, waste products from industrial processes and the like. Biomass is a sustainable feedstock for production of fuels, chemicals, specialty products, heat and electric power which can reduce the nation's dependence on foreign oil.
First generation biofuels, including biodiesel and grain ethanol are produced from food biomass crops. These products are being scrutinized over the perceived competition between food and fuel, net energy output and product incompatibility with existing infrastructure. Second generation biofuels are produced from non-food biomass using biochemical or thermochemical processing.
Biochemical conversion is a multi-step process that first separates biomass into fermentable sugars and lignin. The sugars are biochemically converted into biofuels while lignin is passed through the process unconverted reducing overall biomass conversion efficiencies. Biochemical conversion of biomass requires expensive, complex and selective enzymes and microorganisms which have been proven in the laboratory but not commercially.
Thermochemical conversion of biomass is a robust, high temperature pathway that can process 100% of lignocellulosic biomass. There are three major high temperature pathways: gasification, pyrolysis and combustion. Biomass gasification is a thermal process that produces a gas mixture called synthesis gas or syngas, which can be burned directly to produce heat and power or upgraded into advanced biofuels using Fischer-Tropsch synthesis, fermentation or other upgrading technologies. This type of process, however, requires massive, capital-intense facilities to become economical, and has limited demonstration history using biomass feedstocks (see Anex, R. P. Aden, A.; Kazi, F. K.; Fortman, J.; Swanson, R. M.; Wright, M. M.; et al. Techno-economic comparison of biomass-to-transportation fuels via pyrolysis, gasification, and biochemical pathways. Fuel 2010, 89, S29-S35). Direct biomass combustion or co-firing generates sensible heat that can be used to create electricity at existing facilities, but requires modified equipment to handle biomass. Furthermore, since heat from combustion cannot be easily stored, the heat energy must be used immediately.
Pyrolysis is the thermal decomposition of biomass in the absence of oxygen (see Bridgwater AV. Review of fast pyrolysis of biomass and product upgrading. Biomass and Bioenergy 2011, doi:10.1016/j.biombioe.2011.01.048). It is the only thermochemical process that directly produces a high yielding liquid, called pyrolysis oil (or bio-oil, biocrude oil, wood oil, pyroligneous acid or a number of related terms). Since the liquid product can be stored and transported, the pyrolysis process may be separated from end-use providing increased flexibility over gasification and direct combustion which involve heat and process integration. However, conventional bio-oil has poor properties that limit its application for petroleum replacement products. Poor properties include high water and oxygen content, acidity, instability, complex chemical nature and inability to blend with hydrocarbons (see Qi, Z.; Jie, C.; Tiejun, W.; Ying, X. Review of biomass pyrolysis oil properties and upgrading research. Energy Conversion and Management 2007, 48, 87-92.). Biomass pretreatment, catalytic pyrolysis and post-production upgrading are three schemes being developed to change the product or improve its properties.
Though conventional bio-oil has been combusted for heat and power generation, upgraded into fuels, and extracted into chemicals and materials it is not particularly suitable for any one application. Furthermore, high water content and acidity generally make it a poor fuel, corrosive and incompatible with existing liquid fuel infrastructure. High water content and oxygen make it difficult and expensive to catalytically upgrade conventional bio-oil into transportation fuels. The complex mixture of chemicals in conventional bio-oil make multiple-step extraction and separation processes necessary but time consuming and expensive in producing chemicals and other materials.
One of the challenges in biomass conversion is integrating biomass preparation and conversion with product upgrading and storage.
Another challenge is adapting to and processing different biomass feedstocks while producing a consistent product quality.
Another challenge is incomplete conversion of biomass. Biochemical processing cannot overcome resilient lignin structure reducing overall biomass conversion efficiency.
Another challenge is providing products compatible with existing fuel system infrastructure. Ethanol, biodiesel, syngas and conventional bio-oil among other renewable products have limited use in pipelines, engines, turbines, pumps and other equipment without significant retrofitting, redesign and capital improvements.
Another challenge is the inherent high capital costs for upgrading to hydrocarbon end products. Syngas upgrading using Fischer-Tropsch synthesis requires expensive gas clean-up units, operates at high temperatures, requires large and expensive high pressure vessels with unique metallurgy and involve catalysts. Hydrotreating, hydro-deoxygenation and hydrocracking of conventional bio-oil requires robust catalysts that minimize fouling and coking, are effective in the presence of water and acid, and do not deactivate quickly due to alkali contaminants present in pyrolysis oil. Purchasing or producing hydrogen required for pyrolysis oil upgrading is necessary to remove oxygen and is an expensive part of the process and may increase the carbon footprint of the overall process.
Another challenge is recovering the water soluble and water insoluble (e.g. pyrolytic lignin oligomers and other large molecular weight compounds) phases of conventional bio-oil. While both phases can be used independently and are improved feedstocks for fuels, chemicals and materials versus conventional bio-oil, they must first undergo a time consuming, expensive and intensive extraction process.
Another challenge is producing bio-oil that is versatile and is well-suited for multiple different products with properties that are particularly advantageous for each application.
Another challenge involves producing a stable, low-moisture, low-oxygen content and low-acidity bio-oil that is miscible (easily blends) with hydrocarbons. High acidity, moisture and oxygen content bio-oil does not mix with hydrocarbons, is corrosive and expensive to upgrade. Often expensive, multi-step high pressure and temperature catalytic processes are used to remove oxygen and reduce acidity (see Elliot, D. C., Hu, J., Hart, T. R., Neuenschwander, G. G., Battelle Memorial Institute (2011) Palladium Catalyzed Hydrogenation of Bio-oils and Organic Compounds, U.S. Pat. No. 7,956,224 B2.).
Another challenge is simplifying the chemical composition of bio-oil such that it is compatible with existing products and infrastructure or more easily, efficiently and cost effectively upgraded than conventional bio-oil.
Another challenge involves producing a stable and safe solid biochar co-product that resists spontaneous combustion once exposed to ambient conditions. Conventional biochar also remains a hazard in powder form as it is easily inhaled and easily ignited. Furthermore because it is easily airborne it is difficult to handle and apply to soil or use as a solid renewable fuel.
What are needed in the art are methods and apparatus that combine biomass preparation and conversion with product collection, upgrading and storage to form an integrated fast pyrolysis process that produces stable bio-oil fractions and safe biochar. A preferred integrated, low-capital cost, low-pressure fast pyrolysis process will produce distinct, value-added, stable, bio-oil fractions each having unique properties making them individually superior to conventional bio-oil for direct use or upgrading into fuels, chemicals and materials regardless of biomass feedstock.